Lost Foam Casting (Full Mold Casting) involves placing a plastic pattern of the desired cast part in sand and then pouring molten metal onto the plastic casting causing it to vaporize. The molten metal exactly reproduces the plastic pattern to provide the ultimate casting.
It is known that polystyrene, the major polymer used in this application, produces surface defects when casting iron due to carbon residues left by the polymer. When casting low carbon steel the carbon formed from the polystyrene dissolves in the metal degrading the properties of the cast part. A number of patents describe variations in the Lost Foam Casting process that are intended to minimize the residues left by the polymer after the metal has been poured. Most of these variations involve changing the coating on the pattern or changing the flask in which the casting is made. For example, U.S. Pat. Nos. 4,448,235 and 4,482,000 describe a variable permeability casting designed to avoid entrapment of polymer vapors in the casting. U.S. Pat. No. 3,572,421 describes a flask containing many air breathing holes to allow the escape of polymer degradation products to decrease the formation of carbon. Similarly, U.S. Pat. Nos. 3,842,899, 3,861,447, and 4,612,968 describe the addition of vacuum to the casting flask to aid in the removal of the polymer residues.
The Dow Chemical Company has reported the development of a polymethyl methacrylate foam bead useful to replace polystyrene for the casting process. (Moll and Johnson, "Eliminate the Lustrous Carbon Defect with New Moldable Foam", Evaporative Foam Casting Technology II Conference, Nov. 12-13, 1986, Rosemont, Ill.). Although this polymer reduces residues left on the cast part, it carries with it other disadvantages. The higher glass transition temperature (130.degree. C.) of the polymer causes longer molding cycles when preparing patterns. It uses a Freon blowing agent which has been shown to cause corrosion of molds. It also rapidly gives off a large volume of gas when castings are made. It is very difficult to control the evolution of gas and often the molten metal is blown back out of the flask.
There is still a great need for a polymer that provides the advantages of polystyrene but produces no carbon defects. U.S. Pat. Nos. 4,773,466 and 4,763,715 teach the use of polycarbonate copolymers and terpolymers, respectively, to make patterns for the lost foam casting process. The molded patterns need to be made at a higher density compared to EPS to retain the needed physical properties for a successful casting.
The manufacture of molded articles from expanded thermoplastic polymer particles such as expandable polystyrene beads (EPS) is well known. Two methods are commonly used for preparing molded products from vinyl aromatic monomers. In one method, called herein "the two-step process," the vinyl aromatic monomer, typically styrene, is suspension polymerized to form hard polymer beads, which are isolated and screened to the desired bead size distribution. The sieved polystyrene beads are then resuspended in water, impregnated with a blowing agent--optionally in the presence of a flame retardant, pre-expanded with steam, aged, and molded. This process is illustrated by the teachings of U.S. Pat Nos. 4,028,285 and 4,113,672. In another method, called herein the "one-step process," the vinyl aromatic monomer is suspended-often in the presence of a flame retardant-and with a blowing agent, the partially polymerized mixture is cured without isolation or screening of the beads as in the two-step process. The one-step process for making expandable beads is illustrated by the teachings of U.S. Pat. Nos. 3,755,209, 3,975,327, 4,281,067 and 4,286,071. While the one-step method is simpler, the beads produced tend to have a wider bead size distribution. The method of the present invention encompasses the "one-step process."
In a one-step process the polymerization of styrene can be carried out using benzoyl peroxide as an initiator. The suspension process is carried out in water in a stirred reactor using tricalcium phosphate (TCP) as a suspending agent and Nacconol (sodium dodecyl benzenesulfonate) as a surfactant to keep the styrene droplets from coalescing when they form discrete particles of polystyrene beads. Other systems employ potassium persulfate as an extender. A flame retardant is optionally added and a secondary initiator such as dicumyl peroxide (DiCup), t-amyl peroxy-2-ethylhexyl carbonate (TAEC) or t-butyl peroxy-2-ethylhexyl carbonate (TBEC) is used to reduce the unreacted styrene to less than 1000 ppm in a secondary cure cycle.
At about 70 to 80% conversion of the styrene to polystyrene, pentane as the blowing agent is added over a period of time. Afterward, the second stage of curing occurs at elevated temperatures to finish the polymerization of styrene to polystyrene. The impregnated beads are commonly known as EPS.
An object of the present invention is to completely eliminate carbon defects in lost foam castings. A box pattern is molded from EPS (expandable polystyrene) prepuff, conditioned, and coated with a ceramic finish. The coated EPS patterns are glued in clusters to a sprue which is then placed in a flask, and sand is compacted around it. The box pattern is gated to allow the converging metal, Aluminum 319 alloy at 1350.degree. F., to fill the patterns. The placement of the gating in the box pattern is done to maximize fold defects from converging metal fronts in the casting.